Concerning the sheet stacking apparatus, which is combined with the image forming apparatus, such as a copy machine or a printer, to eject a large number of sheets, many said devices have the sheet sorting function to change a sheet ejecting position for sorting such sheets. Further, many said devices have the sheet aligning function to align the plural sorted sheets.
The sheet stacking apparatuses, including the sheet sorting function and the sheet aligning function, are configured to sort the sheets at two positions, which are at the front side and the rear side of the sheet stacking apparatuses. In case of aligning the sorted sheets, an aligning plate is shifted to a sorting position, where the sheet aligning operation is being conducted.
Before the sheets to be aligned are dispatched to a sheet ejection tray, if a shifting operation of the aligning plate is completed, the aligning operation of the sheets can be properly conducted. However, if the shifting operation of the aligning plate is not completed, the aligning operation of the sheets cannot be properly conducted, and sometimes the sheet aligning plate interferes in ejecting the sheets.
The Unexamined Japanese Patent Application Publication Number 2002-211, 829 discloses a technology to overcome the above problems, in which even though an image forming speed of the image forming apparatus is not lowered, a sheet conveyance speed of the sheet stacking apparatus is lowered, so that the conveyance interval between separating sheets to be sorted is increased. According to this technology, since the conveyance clearance between separating sheets to be sorted is greater, the time period for shifting the aligning plate is certainly secured, whereby the aligning operation can be properly conducted.
However, in the above described technology, in which the sheet conveyance speed is lowered, and the sheet conveyance interval is increased, when plural greater conveyance intervals are to be provided (that is, when the number of sheets to be sorted as a single unit is relatively small, and said units are in the series), the conveyance route of the sheet stacking apparatus becomes full with sheets, so that the sheet stacking apparatus cannot receive the sheets which are ejected from the image forming apparatus. Accordingly, depending on circumstances, the image forming speed of the image forming apparatus needs to be lowered, which is a problem from the point of view of the productivity conducted by the image forming apparatus.
To overcome the above problem, a technology is conceivable in which plural sheets are temporarily stacked on the sheet stacking apparatus, so that the plural sheets remain in the conveyance route of the sheet stacking apparatus, whereby the conveyance interval of the separating sheets to be sorted is increased. However, if the number of sheets to be sorted as a unit is only one, said sheet is included within another unit, that is, said sheet cannot be temporarily stacked alone. Accordingly, in the above technology, the image forming speed of the image forming apparatus must be lowered, which is also a problem from the point of view of the productivity conducted by the image forming apparatus.
Further, considering the sheets to be sorted, if the number of sheets to be sorted as a single unit is plural, the plural sheets are necessary to be aligned in the unit, but if the number of sheets to be sorted as a single unit is singular, said single sheet is not necessary to be aligned in the unit.